Polysaccharides including cellulose are valuable as a biomass resource and used in various applications such as paper, film and fibers.
Unlike synthetic plastics, polysaccharides are generally difficult to melt by heating. Thus they must be once dissolved in solvents before they can be processed. However, it is yet difficult to dissolve polysaccharides. With respect to cellulose, for example, as described in J. Zhou and L. Zhang, Polymer Journal, 32, 10, 866 (2000) and Isogai Ed., Science of Cellulose, Asakura Publishing, 2003, cuprammonium solutions and solutions of cellulose treated with carbon disulfide and sodium hydroxide, known as “viscose,” are on use from the past. Recently, a spinning process using N-methylmorpholine N-oxide was developed. The process requires high-temperature treatment and the recovery of chemicals for reuse and is not advantageous because of the energy and cost spent in forming and processing as well as the recovery.
These problems can be overcome by a process of dispersing polysaccharide in water uniformly and effecting wet milling without breaking the molecular structure for thereby dispersing polysaccharide particles to a state approximate to the molecular level.
The resulting dispersion of polysaccharide is gel-like and can be used as a cosmetic humectant component in toothpaste and cream, timed release gel preparations impregnated with drugs, fragrant preparations, thermal insulating materials and the like. Also films and forms obtained by drying the gel-like material can be utilized as wrapping material and containers. In these uses, as polysaccharides are more finely divided, they become more homogeneous and easier to form into products. If the degree of polymerization is not reduced, the products possess necessary strength and durability.
However, polysaccharides generally have strong hydrogen bonds formed within and between molecules. If pulverizing is effected to such an extent to break such hydrogen bonds, ether bonds that join molecular chains together are often broken at the same time. Pulverizing is accomplished, but the pulverized polysaccharide has a lower degree of polymerization and becomes of poor quality.
For water-insoluble cellulose, JP-B 60-19921 describes a method of imparting a high velocity to a dispersion of cellulose in water under a high pressure difference of at least 3,000 psi (20.6 MPa), followed by an impact for rapid deceleration. In this method, fibrillated cellulose is finely divided to a size of about ¼ or less by repeating about 20 pulverizing cycles. However, if cellulose is pulverized to such a size, the cellulose in a copper ethylenediamine solution has an intrinsic viscosity of 7.55 dl/g which is reduced from the viscosity of 8.83 dl/g prior to the pulverizing. When a degree of polymerization is calculated from this value according to the equation: [η] (ml/g)=1.67×[Dp]0.71 wherein [η] is an intrinsic viscosity and Dp is a degree of polymerization, described in the Cellulose Society Ed., Dictionary of Cellulose, 2000, page 80, it is seen that after 20 pulverizing cycles at 20° C. and 90° C., the degree of polymerization is decreased 20% from the initial. A comparison of a single pulverizing cycle and 10 pulverizing cycles at 20° C. and 90° C. reveals a reduction of at least 10% in degree of polymerization.
JP-B 6-49768 describes a process of subjecting a suspended liquid dispersion of a cellulose derivative, low-substituted hydroxypropyl cellulose dispersed in water to frictional grinding or high-pressure dispersion whereby the low-substituted hydroxypropyl cellulose is frictionally ground into a highly viscous gel-like material. It is not described that the drop of degree of polymerization by frictional grinding is less than 10%.
Although it is known that polysaccharide is processed by frictional grinding, high-pressure impact or wet pulverizing following dispersion, it would be desirable to finely pulverize polysaccharide with minimized breakage of molecular chains while suppressing a reduction in degree of polymerization to less than 10%.